How Saturn’s Moon Got Its Stripes

Researchers from the University of Melbourne have revealed the origins of tiger stripes and a subsurface ocean on Enceladus, one of Saturn’s many moons. In addition to explaining these geological features, the study sheds light on the evolution of planets and guides future space exploration.
 
 This spectacular view is a mosaic of four high resolution images taken by the Cassini spacecraft narrow angle camera on Feb. 16, 2005, during its close flyby of Saturn's moon Enceladus (Credit: NASA/JPL/Space Science Institute)
This spectacular view is a mosaic 
of four high resolution images taken 
by the Cassini spacecraft narrow angle 
camera on Feb. 16, 2005, during its close 
flyby of Saturn’s moon Enceladus. 
(Credit: NASA/JPL/Space Science Institute)

According to the findings, Enceladus’ geological features are the result of the moon’s unusual chemical composition – and not a hot core, as was previously believed. Until now, most astronomers were sure that Enceladus is a lifeless, frozen ice ball; however, in 2006 a water vapour plume was seen erupting from its surface. This event left many scientists intrigued, among them Dr. Dave Stegman, a Centenary Research Fellow in the School of Earth Sciences at the University of Melbourne, who led the study.

“NASA’s Cassini spacecraft recently revealed Enceladus as a dynamic place, recording geological features such as geysers emerging from the ‘tiger stripes’ which are thought to be cracks caused by tectonic activity on the south pole of the moon’s surface,” says Dr. Stegman. Another reason for the academic focus Enceladus is receiving is its luminosity: the moon is one of the brightest objects in our solar system. The cause for this peculiar attribute is its ice-covered surface, which reflects sunlight almost completely.

Saturn has 53 identified moons and its distance from Earth is approximately 1.2 billion kilometers. Therefore, the researchers had difficulty to study Enceladus; in order to thoroughly investigate the ongoing geological activity, Dr. Stegman and his team used computer simulations to virtually explore it.

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One of the measures used was ammonia, which is usually found on Earth as an odorous gas. It has been indirectly observed to be present in Enceladus, and formed the basis of the study, which is the first to reveal the origins of the subsurface ocean. The model proposed in the study reveals that Enceladus initially had a frozen shell composed of a mixture of ammonia and water ice surrounding a rocky core. Over time, as Enceladus interacted with other moons, a small amount of heat was generated above the silicate core; the result was the separation of the ice shell into chemically distinct layers. Afterwards an ammonia-enriched liquid layer formed on top of the core, while a thin layer of pure water ice formed above that.

“We found that if a layer of pure water ice formed near the core, it would have enough buoyancy to rise upwards, and such a redistribution of mass can generate large tectonic stresses at the surface,” says Dr. Stegman. “However, the pure water ice rising up is also slightly warmer which causes the separation to occur again, this time forming an ammonia-enriched ocean just under the surface. The presence of ammonia, which acts as an anti-freeze, then helps keep the ocean in its liquid state.”

The work, which was published in the August issue of the planetary science journal Icarus, has shown that such simulations offer astronomers new ways to study planetary objects, and especially their subsurface. “These simulations are an important step in understanding how planets evolve and provide questions to focus future space exploration and observations,” added Dr. Stegman. “It will hopefully progress our understanding of how and why planets and moons are different to each other.”

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TFOT has previously covered Enceladus’ Subterranean Pockets of Liquid Water, theorized by researchers working on NASA’s Cassini mission, and the Imaging of Saturn and its Rings, created using several images captured by Cassini’s wide-angle camera. Other related TFOT stories include the Mysterious Landscape of Titan, Saturn’s largest moon, and the First Images Ever of Hyperion, one of Saturn’s smallest moons.

For more information about University of Melbourne’s research of Enceladus, see its website.